The variable a minus 10 equals 6. If you substituted a for 16, and subtracted 10, you'd get 6. Substituting the rest of the numbers and subtracting 10 wouldn't give you 6.
a - 10 = 6 in order to find a, you want to get a alone by itself. to do that you add 10 to both sides because it can never be just one side then that will give you a= 10 + 6 which is then a = 16
